Free standing lattice towers have long been used for supporting overhead electrical conductors forming a part of electrical transmission and distribution lines. These towers are relatively inexpensive for the service performed, may be prefabricated and shipped to the site of use for assembly and readily adapt to different terrain conditions. They are made up of standard components cut to size and then for the most part bolted in place at each point where conductor support is dictated by the line design. The towers terminate at the bottom with individual legs that are bolted to a supporting structure therefor which is capable of withstanding both tension, compression and shear loads. Not only does the weight of the tower rest on the leg supports, but tension forces are imposed thereon as winds from varying directions attempt to displace the tower and its load in a direction away from the wind.
In most instances, support for free standing lattice work towers has taken the form of concrete pads poured in place at the four corners of the tower for implacement of the tower legs thereon. Jigs or templates in effect comprising the lowermost bottom section of a tower with members depending therefrom in the same position as the legs of the tower to be fabricated in place are located over the concrete pads prior to setting up of the concrete whereby leg anchors may be embedded in the concrete in required disposition so that upon hardening of the cement, the tower anchors are in precise relative positions for bolting of the tower leg components thereto.
It can be appreciated that this support method for free standing towers can often present construction difficulties not only because of the site of fabrication, but also the necessity of hauling to the tower assembly point means for pouring concrete as well as the jig or tower template required to accurately locate the tower anchors which are embedded into the concrete pads. In remote electrical transmission line sites, the difficulty to transporting cementitious materials to the tower location, building the pads and locating the anchors using a tower jig is a formidable task and substantially increases the cost of the tower construction. Furthermore, in some locations where the ground is of a swampy nature, it is extremely difficult to not only move the materials to the construction area, but equally important, such materials must be moved from tower location to tower location for the next successive installation.
Although it has been suggested that towers may conveniently be supported on screw anchors driven into the ground at each point which is to be a corner of the tower, the procedure has not yet met with commercial success because of the inherent difficulty of driving an anchor into the ground with the required precision that the upper extremity of such anchor is exactly aligned with a respective tower leg for direct bolting of the latter to the anchor shaft or tube. Not only is it impractical to install the screw anchor at an exact required angle with respect to the vertical because of the inability to start such anchor with required accuracy, but equally as important, differences in the stratum of the ground as the anchor is driven into the latter causes deflections of the anchor to one side or the other depending upon the nature of the soil. As a result the upper end of the anchor after completion of the installation thereof, is not necessarily in the precise position required for alignment with a respective tower leg.
It has been suggested in this regard that perhaps the screw anchor foundations can be welded to the tower legs and if necessary intermediate metal shims or connectors may be welded between the screw anchor foundation and the corresponding tower legs to compensate for misalignment of the anchor with the tower leg extension. Although field welding might in many instances solve the problem presented, it is not a satisfactory solution from a construction standpoint because of the difficulty of controlling the quality of the welds under field conditions, the need to transport the necessary welding apparatus to the construction site, the absolute requisite of employing highly skilled welders who can perform quality welding under extremely adverse field conditions, and the reluctance of utilities to support very high voltage electrical conductors on towers where the integrity of the system is dependent on the quality of the welding which has not and cannot be subjected to the same analytical techniques available in shop welding fabrication.